The present invention relates to systems for controlling the transport of an element of indeterminate length and, more particularly, to tape drives adapted for use with computer systems wherein intermittent but precisely located transport of the tape is necessary.
In many computer applications, it is necessary to record or retrieve intelligence from a medium comprising a length of magnetic tape. In performing the recordation or retrieval, it is commonly necessary to periodically stop the advance of the tape abruptly pursuant a command from the computer, and subsequently re-initiate the transportation of the tape so that further portions thereof may be driven past a magnetic read/write head. Typically this movement is accomplished through a capstan drive system wherein the tape is wrapped partially around a capstan, the friction therebetween serving to transfer the tape from a supply reel to a take-up reel. Take-up mechanisms are commonly provided for maintaining a certain amount of slack in the tape on either side of the capstan, to mitigate the stress on the tape which arises from the sudden acceleration and deceleration of the capstan drive.
The sudden acceleration and deceleration of the tape imposes severe requirements upon the capstan control system. Changes in tape speed must be accomplished rapidly in order to maximize the amount of tape available for recording information. It is necessary too that the tape be stopped in a precise position; if it is stopped short access time to the next record on the next start command will be unduly long. If transport of the tape is allowed to continue unduly, insufficient distance will be left to allow the tape to be brought up to speed before reading the subsequent intelligence recorded upon the tape.
Many attempts have been made to provide means for controllably accelerating and/or decelerating transported tape. It has been found, however, that by closely controlling the deceleration characteristics of the system in such a way as to avoid "overrun" of the driven tape, satisfactory transport system operation may be maintained without the necessity for providing complex acceleration waveforms. This is due to the fact that if stopping characteristics are precisely tailored adequate distance can then be left on the tape to allow for a known acceleration characteristic.
Many of the prior art control systems have concerned themselves only with monitoring tape velocity, based on the fact that distance traveled is the time integral of velocity. While this approach has been relatively successful, systems employing it have oftentimes lacked the precision desired in modern-day tape transport mechanisms. It will therefore be appreciated that it would be highly desirable to provide an improved tape transport mechanism which precisely controls the distance traversed by a transported length of tape during deceleration thereof.
It is therefore an object of the present invention to provide improved means for decelerating a transported tape.
It is another object of the invention to provide improved means for precisely regulating the position of a transported tape after it is brought to rest.
It is still another object to provide a tape transport control which will allow the reversing of a capstan drive should an overly long distance be required to stop a transported tape.
Another object of the invention is to achieve improved current limitation in a tape transport system having superior deceleration characteristics.